Process and plant for incinerating waste with preheating of the latter

ABSTRACT

The invention relates to a process for incinerating domestic or industrial waste, in a reactor. The combustion is carried out under pressure and with a supply of pure oxygen in the reactor. In the absence of nitrogen, the vapors resulting from the steam expansion turbine are withdrawn in order to preheat the waste before entry thereof into the reactor, then the remaining gases are condensed with a view to the recovery thereof. A plant carrying out the process mainly includes an incineration line, a vapor circuit, a fuel supply line, a nitrogen circuit and an oxygen circuit.

CROSS-REFERENCE TO RELATED U.S. APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for incinerating household orindustrial waste in a reactor with preheating of the waste by a steamcircuit the steam for which comes from the steam expansion turbine(TRV).

2. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 37 CFR 1.98

The object of the invention is to obtain within the incinerator completecombustion without any unburnt matter, without any troublesome residue,without releasing gas into the atmosphere, in order to protect theenvironment from any pollution.

Another object of the invention is to recuperate the thermal energyreleased, to convert it into electrical energy, and to reuse some ofthis energy within the plant itself. The amount of electrical energyrecuperated is approaching 75%, excluding the energy reinjected into theplant.

BRIEF SUMMARY OF THE INVENTION

These objects are achieved by the invention which consists in a processfor incinerating household or industrial waste in a combustion reactor,characterized in that:

-   -   the combustion is carried out under pressure and with the        addition of pure oxygen to the reactor, and in the absence of        nitrogen,    -   the steam from the expansion turbine is tapped off to preheat        the waste before it enters the reactor,    -   then the remaining gases are condensed in order to recuperate        them.

Furthermore, the method is characterized in that the oxygen needed forcombustion is produced by separating the nitrogen and oxygen from theair, the nitrogen thus produced being used in particular to cool thegases resulting from the combustion of the waste, and the oxygen beinginjected into the reactor at at least one point.

The method as defined allows better destruction of dioxins, unburntmatter, compounds of nitrates, carbonates and phosphates which give riseto oxides.

A plant according to the invention, in order to implement the process,is characterized in that it comprises:

-   -   a feed hopper (TA) having at its inlet and at its outlet a        shutter and comprising means;    -   a feed screw capable of preheating the waste using steam tapped        from the expansion turbine (TRV);    -   an intermediate hopper able to collect the waste reheated in the        feed hopper and introduce it into the top of the reactor, and    -   a reactor equipped with three burners, these being a main burner        (BP), an auxiliary burner (BA) and a catalytic burner (BC)        positioned near the combustion gases outlet, each of these three        burners being fed with fuel by the fuel line (3) and with pure        oxygen by an oxygen production circuit (5), the core of the        reactor comprises a cathode wall based on metals of the tungsten        or tantalum type, chosen for their refractory nature.

As a preference, the oxygen is produced by separating air into nitrogenand oxygen.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionwhich follows, given with reference to the following attached figures.

FIG. 1 is a schematic view of an overview of a waste incineration plantaccording to the invention.

FIG. 2 is a schematic view of a diagram of the water vaporizationcircuit of the plant.

FIG. 3 is a detailed schematic view of a diagram of the feed hopper andof the feed screw according to the invention.

Reference is made first of all to FIG. 1 which shows a plant in itsentirety, this plant essentially comprising:

-   -   an incineration line 1,    -   a steam circuit 2,    -   a fuel feed line 3,    -   a nitrogen circuit 4, and    -   an oxygen circuit.

DETAILED DESCRIPTION OF THE INVENTION

Incineration line (1).

Trucks containing the waste that is to be destroyed are unloaded undergravity into a feed hopper (TA) the outlet of which is equipped with ashutter (OF1).

A feed screw (VA) receives the waste from the feed hopper (TA) andconveys it and tips it into an intermediate hopper (TI) via an inletsituated at the top of said hopper (TI) and equipped with a shutter(0F2).

The feed screw (VA) also allows the waste to be preheated as will beexplained later on. The intermediate hopper (TI) has a central bottomoutlet equipped with a shutter (0F3) and via which it loads the wasteunder gravity into an inlet chute opening onto an opening shutter (0F4)of a combustion reactor (RC) and at the top thereof.

For preference, the hopper (TI) is pressurized at a temperature close to900° C. to accelerate the reforming of and removal of halides from thePOPs in order to facilitate the expulsion of waste to the combustionreactor. Advantageously, the pressurizing will be performed byintroducing a mass of steam at high pressure and high temperature inexcess of 1000° C. into the hopper (TI) via at least one appropriateorifice. This gaseous mass at high pressure will have the advantage offluidizing the mass of waste present in the hopper TI and as a result ofmaking it easier to cause it to flow to the combustion reactor (RC). Inorder to prevent any leak of gaseous mass from the intermediate hopperto the hopper TA while said intermediate hopper is being filled, theorifice via which the gaseous mass is introduced will be associated witha remote-controlled valve. This valve will be in the position of closingoff the orifice when the hopper loading hatch is in the open positionand will be in the position in which the orifice is open when theloading hatch of the intermediate hopper is in the closed position.

It will be possible to use several hoppers each in turn communicatingwith the feed hopper and each in turn communicating with the reactor RC.This arrangement will allow one intermediate hopper to be filled fromthe feed hopper when the other or one of the other hopper(s) is in theprocess of unloading into the reactor (RC).

It will be possible to incorporate, after the feed hopper, a tank thatwill allow the waste to be mixed with an additive based on sodiumhydroxide or on potassium hydroxide in order, at temperatures close to200° C., to neutralize acids in a first phase and the halides bound upin the inorganic molecules.

Halides present in the POPs (persistent organic pollutants) will beeliminated or fixed using alkali metal hydroxides in the intermediatehopper at temperatures close to 1000° C.

The combustion of waste produces fly ash and gases. The fly ash drops tothe bottom of the reactor (RC) and then into a bottom ash hopper (TC)situated under the reactor (RC). This bottom ash hopper conveys the flyash to an ash cooling recuperator (RCE) via a shutter (0F5). Therecuperator RCE mixes the fly ash with water and initiates reactionsbetween the oxides and the water to form soluble hydroxides. Next, theinsoluble fly ash is tipped into a truck which takes it away (1 c).

All the fly ash, except for the air pollution control residue (APCR)will be processed using water at temperatures of between 200 and 400°C., at the outlet from the reactor. No additional energy is needed toobtain these temperatures because the dilution of alkali metal oxides isexothermal.

A processing circuit enables soluble waste to be separated frominsoluble waste, the insoluble waste being sent for sedimentation andsome of the soluble content will crystallize and be able to be reused.The soluble part will be reintroduced into the feed hopper following theseparation of the salts of the halides, and of the sulfates of potassiumand of sodium.

The combustion reactor (RC) is equipped with refractory bricks for goodthermal insulation and a cathode wall based on tungsten or tantalum atthe heart of the reactor ensures that the waste is burnt at very hightemperatures ranging between 1500-3000° C., and is so using threeburners (BP, BA, BC) fed with fuel and with oxygen and respectively:

-   -   a main burner (BP) positioned at the base of the combustion        reactor (RC),    -   an auxiliary burner (BA) positioned in the middle part of the        combustion reactor (RC),    -   a catalytic burner (BC) positioned at the upper part of the        reactor and near the outlet of the gases, combustion gases 5        completing and optimizing the combustion.

The primary and auxiliary burners operate with an excess of oxygen at arate of reaction 10 to 20 times higher than the habitual speed ofcombustion reactions.

The reactor (RC) is designed to operate at constant high pressure andconstant high temperature, and its inlets and outlets therefore consistof hatches that constitute heat shields and provide sealing.

The combustion reactor will preferably be a thermal oxidation reactor(TOR).

The hoppers also operate under pressure and consist of air locks withtheir inlet and outlet shutters.

Safety valves CE1 and CE2 are also provided in the reactor and in theintermediate hopper.

The shutters OF1 to OF5 can be actuated by motors external to theelements to which they are fitted. The motors will be of any known type.Without implying any limitation, they could consist of remote-controlledelectric, hydraulic or pneumatic cylinder actuators.

The combustion gases are capped from the outlet (1 a) at the top of thereactor (RC) and sent through a pipe (SGC) to a particulate filter (PF)and then into heat exchangers ECT1, ECT2 toward an expansion turbine(TRGC).

The expansion turbine (TRGC) is advantageously associated with anelectric energy generator (GE3) and so some of the heat energy of thecombustion gases is thus converted into electricity.

The water vapor is condensed and the gaseous oxides are removed (1 b) to(CGC). Some of the water from CV2 is reintroduced into the compressor(7), having passed through an osmotic filter.

Steam Circuit (2)

Advantageously, some of the condensed water can be recuperated andvaporized into the form of high-pressure and high-temperature dry steamto form the high-pressure gaseous mass introduced into the intermediatehopper. Thus, upon leaving the condenser (CV1) the water will be bled toa heat exchanger 6 where it is vaporized into the form of high-pressuredry steam. Advantageously, the heat exchanger may consist of a tubebundle in thermal contact with the reactor (RC) to recuperate some ofthe heat given off by the latter, thereby stabilizing the temperatureinside the reactor, this heat being used to vaporize the water and mostof the steam being directed to an expansion turbine (TRV), anotherproportion of it being injected into a pipe (TI).

The steam leaving the TRV is introduced into a preheating device (SP)incorporated into an endless feed screw (VA) provided between the feedhopper (TA) and the intermediate hopper (TI). This feed screw (TA)comprises a longitudinal shaft (2 d) on which a screw thread (2 c) ismounted. A drive member of any known type will be coupled to the shaftof the screw.

The preheating device (SP) is preferably, but nonlimitingly, that ofFIG. 3 which consists of a screw thread (2 c) in the form of a box witha gas inlet downstream to the screw and a gas outlet upstream to thescrew, the gas outlet being between the screw thread and the inletshutter (OOF1). The upstream and downstream inlets are each formed of ablind axial drilling made in the shaft (2 d) at a corresponding end andof a radial drilling made in said shaft and opening, at one end, intothe blind axial drilling and into the box form that the screw thread (2c) exhibits. The steam is injected axially into the start of the screwthread and heats up the waste as it travels along the screw thread, thenleaves the thread to be sent to a first condenser (CV1), having passedthrough a compressor 7.

As a preference, a compressor 7 may be positioned upstream of theexchanger 6 to pressurize the water and create at this point a backpressure that prevents the reflux of steam to the condenser (CV1).

It may be pointed out that the circulation of steam through the screw iscountercurrent with respect to the progress of the waste carried by thisscrew.

The condensers (CV1, CV2, CGC) are of the conventional type withtube-type heat exchangers through which a refrigerant from anevaporator-type refrigeration device (EFF) passes.

The tapped-off combustion gases are gases which are oxidized andstabilized without dioxin and without unburnt matter in the duct SGC.Some of their heat energy is converted into electrical energy in agenerator associated with the turbine (TRGC) and most of the energy isused to heat up the nitrogen.

Following cooling using nitrogen, the combustion gases are conveyed toECT1 and ECT2. These gases are condensed and then introduced into theturbine TRGC which converts the energy of the combustion gases back intoelectrical energy. Following expansion, the gases are separated from thesteam, because the latter condenses.

This water circuit (1 d) also contains at least one means (for exampleusing osmosis filtration) of inerting the water that has been condensedin the condenser (CV2).

Fuel line (3)

To feed fuel along a line (3), provision is made for the fuel to betaken from a tank (RCA) and injected under high pressure into each ofthe three burners, namely the main burner (BP), the auxiliary burner(BA) and the catalytic burner (BC).

Nitrogen circuit (4)

A bank of air compressors (BCA) compresses the atmospheric air from onebar to about 300 bar, this air being cooled after each compression stagein heat exchangers using the refrigerant conveyed along a pipe (4 a)from the refrigeration device (EFF) already mentioned.

A turbocompressor (TCA) expands the air from 300 bar to about 50 bar,this expansion being accompanied by a cooling of the air from −43°(approximately, on leaving the heat exchanger of the final compressionstage) down to −134° approximately, thus allowing the gaseous nitrogento be separated from the liquefied oxygen inside an air expansion vessel(BDA).

The same turbocompressor (TCA) recompresses the gaseous nitrogen fromabout 50 bar to about 280 bar, liquefies some of it in RAL and sends theremainder of the gaseous nitrogen to a nitrogen tank RAG.

The nitrogen is then sent from the tank (RAG) to a heat exchanger (CFF2)where it is heated back up to about 61° C. then sent into the tube-typeheat exchangers ECT1, ECT2 in order to cool the combustion gases to 200°C. The nitrogen is heated back up to about 900° C. countercurrent to thecombustion gases. The nitrogen is then sent into the nitrogenrecuperation turbine (TRA) associated with an electric generator GE3. Inthis way, the nitrogen is used to recuperate heat energy, which energybecomes converted into electrical energy by the generator GE3.

The nitrogen separation circuit that has just been described by way ofnonlimiting example is intended to avoid the encumbrance associated withnitrogen the atmospheric air content of which is 78%, and associatedwith the production of unwanted Nox.

Oxygen circuit (5)

A paramagnetic separator separates the liquid oxygen from the gaseousnitrogen leaving the expansion vessel BDA.

The liquid oxygen is sent to a liquid oxygen tank (ROL). Followingstorage, it is preheated in an exchanger CFF2 from −134° toapproximately 0° at which it turns into a gas and is directed to thereactor RC to feed each of the three burners (BP, BA, BC).

The oxygen feed to the burners encourages complete combustion of thewaste.

An additional catalytic burner (not depicted), also fed with oxygen,also allows dioxin molecules to be broken down and the elimination ofany unburnt matter.

The means just described for separating the air into oxygen and nitrogenis used in preference for high-capacity incinerators according to theinvention.

For incinerators according to the invention, but which are of lowcapacity, it maybe preferable to use air separators operating usingmembrane filters to separate the oxygen and the nitrogen, this type ofseparator making it possible to obtain the oxygen and the nitrogendirectly in gaseous form.

The advantages afforded by this novel type of incinerator are asfollows:

-   -   to incinerate the same amount of waste, the volume of the        incinerator is markedly lower than that of the incinerators        currently in use,    -   the use of oxygen in place of air reduces the volume of oxidizer        by the 79% of nitrogen contained in atmospheric air.        High-pressure operation within the incinerator speeds up the        rate of combustion of waste in the presence of oxygen, the        absence of nitrogen allowing direct contact with the oxidizer,    -   the recuperation of the nitrogen from the separation of air        makes it possible in high-capacity incinerators to produce        energy using the recuperation turbines coupled to an electric        generator,    -   because the incinerator operates under pressure, the combustion        gases produce energy through the use of recuperation turbines        each coupled to an electric generator,    -   given the operating characteristics of this novel type of        incinerator, the investment required to incinerate the same        amount of waste is lower than that required with present-day        incinerators,    -   the closed-circuit operation achieved thanks to the        recirculation of the oxidized gases avoids any discharge of gas        into the atmosphere,    -   the plant can incinerate all kinds of waste including asbestos        and drilling sludge for example.

1. A process for incinerating household or industrial waste in areactor, characterized in that , said process comprising the steps of:combusting waste under pressure and with addition of pure oxygen to thereactor, and in the absence of nitrogen; tapping off steam from anexpansion turbine to preheat the waste before the waste enters thereactor; and condensing remaining gases in order to recuperate theremaining gases.
 2. The method as claimed in claim 1, wherein the oxygenneeded for combustion is produced by separating the nitrogen and oxygenfrom the air, the nitrogen thus produced being used to cool the gasesresulting from the combustion of the waste, and the oxygen beinginjected into the reactor at at least one point.
 3. A plant forincinerating household or industrial waste, said plant comprising: areactor with at least one burner fed by a fuel feed line; a feed hopperhaving a shutter at an inlet and outlet thereof and comprising means forpreheating the waste using steam tapped from an expansion turbine; anintermediate hopper to collect the waste preheated in the feed hopper,introducing the preheated waste into the top of the reactor; and areactor equipped with three burners, the three burners being comprisedof a main burner, an auxiliary burner and a catalytic burner positionednear the combustion gases outlet, each of the three burners being fedwith fuel by the fuel line and with pure oxygen by an oxygen productioncircuit.
 4. The plant as claimed in the preceding claim, characterizedin that the claim 3, wherein said means for reheating the wastecomprises a screw thread in the form of a box with a gas inletdownstream to the screw and a gas outlet upstream to the screw, the gasoutlet being at the other end between the screw thread and the inletshutter.
 5. The plant as claimed in claim 3, further comprising: a steamcircuit attached to the walls of the reactor.
 6. The plant as claimed inclaim 5, wherein the steam recovery circuit comprises at least onecondenser.
 7. The plant as claimed in claim 3, further comprising: anair separator operating using membrane filters to separate the oxygenand the nitrogen.
 8. The plant as claimed in claim 3, furthercomprising: an air separator being comprised of a bank of aircompressors and a turbocompressor, separating gaseous nitrogen fromliquefied oxygen.
 9. The plant as claimed in claim 8, wherein oxygenproduction circuit comprises, after the air separator and theturbocompressor, an expansion vessel, a liquid oxygen storage tank, anexchanger, wherein the oxygen is gasified to be fed to each of the threeburners.
 10. The plant as claimed in claim 8, further comprising: afterthe air separator and the turbocompressor, an expansion vessel, anitrogen tank, three heat exchangers, and a tube-type heat exchanger anda nitrogen recuperation turbine.
 11. The plant as claimed in claim 5,wherein the expansion turbine is associated with an electric generator.12. The plant as claimed in claim 3, wherein the hopper is pressurizedto facilitate the expulsion of waste to the combustion reactor.
 13. Theplant as claimed in claim 12, wherein pressurizing of the intermediatehopper is performed by introducing a gaseous mass under high pressureinto said hopper via at least one appropriate orifice.
 14. The plant asclaimed in claim 13, wherein an orifice via which the gaseous mass isintroduced into the intermediate hopper is associated with aremote-controlled valve in order to prevent any leakage of gaseous massfrom the intermediate hopper to the hopper while said intermediatehopper is being filled.
 15. The plant as claimed in claim 12, whereinthe gaseous mass is high-pressure dry steam.
 16. The plant as claimed inclaim 4, wherein gases leaving the screw thread of the screw are sent toa first condenser is to condense the, condensing water vapor containedin the combustion gases, having passed through a compressor.
 17. Theplant as claimed in claim 15, wherein water condensate leaving thecondenser is bled off to a heat exchanger, the water condensate beingvaporized to the form of high-pressure dry steam.
 18. The plant asclaimed in claim 17, wherein the heat exchanger of comprises a tubebundle in thermal contact with the reactor to recuperate some of theheat given off thereby, this heat being used to vaporize the water. 19.The plant as claimed in claim 18, further comprising: a compressorpositioned upstream of the exchanger to pressurize the water and tocreate at this point a back pressure which prevents the reflux of steamtoward the condenser.
 20. The plant as claimed in claim 3, wherein,after cooling using nitrogen, the combustion gases are conveyed andcondensed then introduced into the turbine, and wherein, afterexpansion, the gases become separated from the condensed water.